Coherency Inversion Research Articles

Characterizing near-surface structures at the Ostia archaeological site based on instantaneous-phase coherency inversion

Traditional least-squares full-waveform inversion (FWI) suffers from severe local minima problems in the case of the presence of strongly dispersive surface waves. In addition, recorded wavefields are often characterized by amplitude errors due to varying source coupling and incorrect 3D-to-2D geometric-spreading correction. Thus, least-squares FWI is considered less suitable for near-surface applications. We introduce an amplitude-unbiased coherency measure as a misfit function that can be incorporated into FWI. Such coherency has been used previously in phase-weighted stacking to enhance weak but coherent signals. The benefit of this amplitude-unbiased misfit function is that it can extract information uniformly for all seismic signals (surface waves, reflections, and scattered waves). Using the adjoint-state method, we show how to calculate the gradient of this new misfit function. We validate the robustness of the new approach using checkerboard tests and synthetic data contaminated by random noise. We then apply the new FWI approach to a field data set acquired at an archaeological site located in Ostia, Italy. The goal of this survey is to map the unexcavated archaeological remains with high resolution. We identify a known tumulus in the FWI results. The instantaneous-phase coherency FWI results also establish that the shallow subsurface under the survey lines is quite heterogeneous. The instantaneous-phase coherency FWI of near-surface data can be a promising tool to image shallow small-scale objects buried under shallow soil covers, as found at archaeological sites.

2D Seismic Imaging Improvement through Prestack Depth Migration (PSDM) Method

The quality of seismic is important for interpretation. Prestack Depth Migration produce better quality of seismic imaging. The seismic generated through PSDM method has better seismic reflector and geological structure appearance compared to Prestack Time Migration (PSTM) method. Accurate interval velocity modeling is a key in PSDM process, involving dix transformation, coherency inversion, and tomography. Comparison between PSTM and PSDM show that PSDM offer better imaging for interpretation because PSDM has better seismic reflector continuity and good geological appearance.

A seismic study to investigate the prospect of gas hydrate in Mahanadi deep water basin, northeastern continental margin of India

The presence of gas hydrates, one of the new alternative energy resources for the future, along the Indian continental margins has been inferred mainly from bottom simulating reflectors (BSR) and the gas stability zone thickness mapping. Gas hydrate reserves in Krishna Godawari Basin have been established with the help of gas-hydrate related proxies inferred from multidisciplinary investigations. In the present study, an analysis of 3D seismic data of nearly 3,420 km2 area of Mahanadi deep water basin was performed in search of seismic proxies related with the existence of natural gas hydrate in the region. Analysis depicts the presence of BSR-like features over a large areal extent of nearly 250 km2 in the central western part of the basin, which exhibit all characteristics of a classical BSR associated with gas hydrate accumulation in a region. The observed BSR is present in a specific area restricted to a structural low at the Neogene level. The coherency inversion of pre-stack time migration (PSTM) gathers shows definite inversion of interval velocity across the BSR interface which indicates hydrate bearing sediments overlying the free gas bearing sediments. The amplitude versus offset analysis of PSTM gathers shows increase of amplitude with offset, a common trend as observed in BSR associated with gas hydrate accumulation. Results suggest the possibility of gas hydrate accumulation in the central part of the basin specifically in the area of structural low at the Neogene level. These results would serve as preliminary information for selecting prospective gas hydrate accumulation areas for further integrated or individual study from geophysical, geological, geochemical and microbiological perspectives for confirmation of gas hydrate reserves in the area. Further, on the basis of these results it is envisaged that biogenic gas might have been generated in the region which under suitable temperature and pressure conditions might have been transformed into the gas hydrates, and therefore, an integrated study comprising geophysical, geological, geochemical and microbiological data is suggested to establish the gas hydrate reserves in Mahanadi deep water basin.

The success of pre-stack depth migration over the Anama structure in the Papuan Foreland Basin, PNG: a case history

The successful application of Pre-Stack Depth Migration (PreSDM) over the Anama structure has been demonstrated by the recent drilling of the Anama-1 well. The effectiveness of traditional depth conversion methodologies is limited in the Papuan Gulf Area due to significant lateral velocity variations introduced by thick Tertiary carbonates, channelling and shallow gas accumulations in Pliocene-Recent sediments. As a result the validity of Mesozoic structural closures previously drilled in the offshore Papuan Foreland Basin is highly questionable. Sparse well control, and stacking velocities significantly affected by raypath distortions, hinders the definition of accurate velocity fields. In an attempt to resolve these limitations and improve the accuracy of the interpreted velocity fields, PreSDM was applied to a modern 2D seismic grid acquired in 1998. Detailed velocity interpretation was conducted using coherency inversion across the seismic grid over the entire Anama structure. The resultant velocity model was tied across the seismic data set prior to generating the PreSDM sections. The final depth interpretation was completed on a workstation using the PreSDM sections. The PreSDM based depth map was used to locate the Anama-1 well. A depth closure was identified with the interpretation of the PreSDM sections where no valid closure was present in the time domain. Drilling results from Anama-1 showed the maximum error for predicted tops based on the PreSDM processing was within 1.2% while the error for the predicted top target sand was less than 0.5%. PreSDM processing is principally aimed at improving imaging and lateral continuity. However by tying the velocity model used in the PreSDM processing across a seismic grid it was possible to improve the accuracy of absolute depths without detrimentally affecting both imaging and lateral continuity. In this case history it was possible to use PreSDM as a predictive tool for absolute depths.

Detection of shallow objects using refracted and diffracted seismic waves

The detection of targets such as tunnels, karst, mines and other local heterogeneities is an important but difficult task in subsurface studies. In this paper a new methodology for determining a near-surface velocity–depth model is described. Common shot gathers and first breaks of refracted seismic waves are used as input data. Low-frequency components of the model are constructed by the intercept time method and coherency inversion. Detection of high-frequency velocity variations is carried out by refraction tomography using the low-frequency model as background. Diffracted waves contain valuable information regarding both the structure and composition of seismic media, especially in cases where target size is comparable to seismic wavelength. A diffraction stack serves as an additional tool for delineation of local scattering objects. The methodology was applied to define tunnel position and to search for the karst formations.

3-D reservoir characterization with horizon visualization and inversion animation

The Echo Springs Field and surrounding Wamsutter Development Area in the Greater Green River Basin (GGRB) of Wyoming are currently being developed with over 600 mi2 of 3-D seismic coverage using coherency data, visualization, and inversion animation. Reserves in this area, Amoco’s second largest natural gas property in North America, are estimated at 1.4 trillion ft3. The development team’s goal is to increase these reserves to 2 trillion ft3 by the year 2000 by drilling 40 additional wells per year.

Velocity-Depth Model Estimation for a Subsalt Target from the Southern Gas Basin of the North Sea: ABSTRACT

The Southern Gas Basin of the North Sea has been subjected to extensional tectonics, primarily in the east-west direction. Subsequent occurrence of the salt diapirism gave rise to the presence of complex structures. By doing a depth-domain analysis of a 3-D seismic survey data from an area in the Southern Gas Basin, we delineated the structural geometry of the top Rotliegendes formation beneath the complex Zechstein diapiric formation. This required an accurate estimate of the velocity-depth model above the Zechstein diapiric formation and removal of its deleterious effect on the underlying Permian sands of Rotliegendes and deeper targets. We conducted a layer-by-layer depth-domain analysis, and used coherency inversion to estimate layer velocities and 3-D poststack depth migration to delineate reflector geometries down to top Zechstein. We verified the accuracy of the velocity-depth model for the overburden above Zechstein by analyzing image gathers from prestack depth migration. We then analyzed constant-half-space image-gather stacks to estimate velocities for the substratum including Zechstein and the underlying Carboniferous sequence. Finally, we performed 3-D prestack depth migration to delineate the base Zechstein - top Rotliegendes geometry. This study demonstrates the need for depth-domain analysis of seismic data to derive accurate structure maps for targets beneathmore » complex structures associated with salt and overthrust tectonics. The final output from depth-domain analysis -- a velocity-depth model, can then be used as a canvas for a reservoir model.« less

Can the aeromagnetic method yield more information in the offshore oil and gas exploration?

The detection of targets such as tunnels, karst, mines and other local heterogeneities is an important but difficult task in subsurface studies. In this paper a new methodology for determining a near-surface velocity–depth model is described. Common shot gathers and first breaks of refracted seismic waves are used as input data. Low-frequency components of the model are constructed by the intercept time method and coherency inversion. Detection of high-frequency velocity variations is carried out by refraction tomography using the low-frequency model as background. Diffracted waves contain valuable information regarding both the structure and composition of seismic media, especially in cases where target size is comparable to seismic wavelength. A diffraction stack serves as an additional tool for delineation of local scattering objects. The methodology was applied to define tunnel position and to search for the karst formations.

Accuracy study of velocity estimation in 3-D layered models

Velocity estimation is examined in 3-D layered structures formed by plane and curved interfaces. The applied technique of coherency inversion tests the layer velocity through the repeating sequence of ray migration/coherency measurement. The reconstructed velocity‐depth model fits zero‐offset reflection times and maximizes semblance on input common midpoint (CMP) gathers. The correctness of layer velocity analysis disregarding the three‐dimensionality of the structures is under consideration. Using the 2-D coherency inversion technique, velocity is correctly determined in the upper layer of the examined structures. Two‐dimensional analysis in the deeper layer gives biased velocity estimates. The errors in the 2-D velocity estimates vary with the profile azimuth and appear in the form of the apparent velocity anisotropy. The inaccuracy of 2-D velocity estimation is analytically considered for the profile oriented along the refractor strike direction. The derived equation relates the velocity error to structure geometry and to the velocity contrast above and below the refractor. Three‐dimensional velocity analysis in the examined structures reveals that the layer velocity resolution is affected by the refractor shape. Below the convex refractor the velocity resolution deteriorates compared with that below the plane.

Interpretation of velocity estimates from coherency inversion

Velocity estimation by coherency inversion provides a velocity‐depth macro‐model of the subsurface. The accuracy of this model is critical for imaging techniques such as depth migration and inversion. Therefore, the reliability of seismic interpretation is strongly dependent on the accuracy of the derived model. Uncertainties in the velocity and depth estimates for the model can be calculated using a combination of the response surface technique and the Dix equation. In the layer‐stripping strategy, the method takes into account the local errors connected mainly to the current layer and the global errors connected to the derived overburden. Examples using synthetic and real data show that the coherency inversion produces accurate macro‐models.

ELASTIC PARAMETER ESTIMATION BY COHERENCY OPTIMIZATION1

We present a method for estimating P- and S-velocities within defined layers (macromodel), using only kinematic properties (i.e. traveltimes) of the wavefield. The method does not require identification of mixed-mode events on prestack or post-stack data. After obtaining a Vp-depth model by coherency inversion, S-velocities are determined by coherency optimization along computed traveltime curves for mixed-mode events on prestack data. Since the method does not involve any dynamic wavefield computations, a simple ray-tracing algorithm is used to solve the forward problem. The simplicity of the scheme, together with the ability to apply it locally, makes it highly suitable for interactive use. Results of this method may be used to detect Poisson's ratio anomalies within or between layers and may serve as an initial model for more complicated elastic inversion algorithms.

Reference velocity model estimation from prestack waveforms: Coherency optimization by simulated annealing

Coherency inversion, which consists of maximizing a semblance function calculated from unstacked seismic waveforms, has the potential of estimating reliable velocity information without requiring traveltime picking on unstacked data. In this work, coherency inversion is based on the assumption that reflectors’ zero‐offset times are known and that the velocity in each layer may vary laterally. The method uses a type of Monte Carlo technique termed the generalized simulated annealing method for updating the velocity field. At each Monte Carlo step, time‐to‐depth conversion is performed. Although this procedure is slow at convergence to the global minimum, it is robust and does not depend on the initial model or topography of the objective function. Applications to both synthetic and field data demonstrate the efficiency of coherency inversion for estimating both lateral velocity variations and interface depth positions.

A Non-Linear Optimization Technique for the Inversion of Long Range Refraction Profiles

The problem of interpreting long range crustal refraction profiles is usually addressed by a conventional estimation of the velocity-depth model from the travel time data followed by iterative forward modelling. In principle, the objective of this technique is to find the crustal model which gives the best agreement between the calculated and observed travel time data. Recent advances in the design of ocean bottom seismometers (OBS) have facilitated the use of a large number of closely spaced OBS along such profiles. This has increased the resolution of the technique and the constraints put on the calculated models but at the same time increased considerably the complexity of the interpretation process and the computation time. In order to reduce the tedious iterative interpretation process to manageable proportions, we propose to apply a development of the coherency inversion method, suggested for use in exploration reflection interpretation by Landa et al. (1987) and Koren et al. (1987), to the evaluation of long range crustal profiling data. This development is designed for the estimation of the velocity sequences and the geometry of the interfaces using selected record sections for a given profile. We are concerned with two-dimensional heterogeneous media and data which contain arrivals of reflected and refracted waves. The method is based on an iterative algorithm producing a model which maximises some measure of coherency; the latter is computed within a time gate for a record section along travel times generated by tracing rays through the model. This algorithm has the advantage that event picking on the record sections is avoided and it is not based on curve fitting of hyperbolic approximations of the arrival times.